You have to read a long way through this blog article from Microsoft to find it..
http://blogs.windows.com/windowsexperience/2015/10/29/making-it-easier-to-upgrade-to-windows-10/
But it says;
You will have to be careful if you don't want it installing as windows now updates automatically for most people.
Also of note is that you can disable this default behaviour by setting your internet connection as metered.
http://blogs.windows.com/windowsexperience/2015/10/29/making-it-easier-to-upgrade-to-windows-10/
But it says;
Early next year, we expect to be re-categorizing Windows 10 as a “Recommended Update”. Depending upon your Windows Update settings, this may cause the upgrade process to automatically initiate on your device. Before the upgrade changes the OS of your device, you will be clearly prompted to choose whether or not to continue. And of course, if you choose to upgrade (our recommendation!), then you will have 31 days to roll back to your previous Windows version if you don’t love it.
You will have to be careful if you don't want it installing as windows now updates automatically for most people.
Also of note is that you can disable this default behaviour by setting your internet connection as metered.
If you are on a metered connection on Windows 7 or Windows 8.1, then you have the option of turning off automatic updates. We strongly discourage this in today’s connected world because of the constant risk of internet threats. If you choose to do so, we recommend manually checking Windows Update frequently, perhaps when you are on a non-metered network and consider the installation of all “Recommended” and “Important” updates. Windows 10 will not automatically download updates on a metered connection unless there is a security issue addressed within the update